This invention relates to a fission-fragment induced desorption time-of-flight mass spectrometer. More particularly, this invention relates to a fission-fragment induced desorption time-of-flight mass spectrometer wherein sample ions are desorbed from the same surface of the sample impinged upon by desorption-inducing fission fragments.
Mass spectrometry is an analytical technique for determining the chemical and isotopic composition of a sample of interest. In a typical mass spectrometer, a small quantity of the sample is ionized into molecular ions and quasimolecular ion fragments which are volatilized. The masses of these volatilized ions and ion fragments are then determined, and from this information the chemical and isotopic composition of the original sample may be deduced.
One method for determining ionic masses is known as time-of-flight mass spectrometry (TOFMS). By this method sample ions of different masses but having the same kinetic energy travel the length of a flight region which is free of any electrical or magnetic fields. Ions with different masses travel the length of the flight region with different velocities such that lighter ions traverse the flight region faster than heavier ions. Detectors at each end of the flight region signal the precise start time and arrival time of each ion. The mass of each ion may then be determined from the time required for the ion to traverse the length of the flight region and the known kinetic energy. A mass spectrum is developed by compiling the flight times of many desorbed ions and analyzing the data by computer.
It is desirable to ionize and volatilize the sample with minimal fragmentation of the sample molecules so that the ionic masses determined will correspond as closely as possible to the masses of the original sample molecules. This may be accomplished by means of desorption. Desorption is a process whereby a highly energetic particle strikes a solid sample, transferring a large quantity of energy to a very localized area of the sample and causing ejection of minute amounts of the sample from the surface of the localized area. In fission fragment induced desorption (FFID), the high energy particles are nuclear fission fragments. Although desorption is a very localized and highly energetic process with temperatures estimated to reach as high as 66,000.degree. K., the desorption occurs so quickly that energy is not transferred to the vibrational modes of the desorbed molecules. As a result, many chemical bonds that would normally break at such high temperatures remain intact during desorption, and fragmentation of the desorbed sample molecules is minimized.
Desorption-inducing nuclear fission fragments may be emitted from a radioactive source such as .sup.252 Cf. The .sup.252 Cf nucleus fissions spontaneously into two fission fragments which travel in almost exactly opposite directions. Thus each fission fragment which strikes a sample to induce desorption has a complementary fragment which can be detected and used as a time zero marker. By continuing to expose the sample to the fission fragment source, desorption is automatically repeated many times per second, depending on the strengh of the .sup.252 Cf source.
Fission fragment induced desorption TOFMS is a useful and versatile technique, but it has heretofore been subject to limitations, particularly with respect to the types of samples that can be analyzed and preferred methods of sample preparation. These limitations occur in part because of the configuration in the mass spectrometer of the fission fragment source, sample, and flight region. In prior art devices, these components have been in a linear configuration with the sample between the source and the flight region. In these spectrometers the sample is a thin film supported upon a metal foil, with the metal foil facing the source and the exposed sample facing the flight region. In this arrangement a fission fragment emitted from the radioactive source must penetrate the metal foil and the sample and exit the opposite face of the sample to induce desorption of sample particles from the opposite face, whereupon the desorbed sample particles pass into the flight region. This configuration has been used to maximize the number of fission fragments which strike the sample. It may be seen that in this arrangement the sensitivity of the device is dependent on the thinness and uniformity of the sample and hence of the method of sample preparation. Since fission fragment induced desorption is a surface phenomenon, the desorption yield is proportional to the area of foil surface actually covered by sample molecules. Samples are typically deposited by solvent evaporation to produce a thin film on the metal foil. This produces non-uniform samples and up to 100 .mu.g/cm.sup.2 must be deposited to adequately cover the surface. Electrospray techniques allow the deposition of more uniform samples. Nevertheless, not all samples of interest are amenable to dissolution and thin-film depostion by these techniques. For example, it is extremely difficult to prepare samples of insoluble metal oxides for isotopic analysis by these methods.
Fission fragment induced desorption TOFMS is described by R. D. Macfarlane and D. F. Torgerson in Science, Vol. 191, Mar. 5, 1976, pp 920-925, and by the same authors in International Journal of Mass Spectrometry and Ion Physics, Vol. 21, 1976, pp 81-92, Elsevier Scientific Publishing Company. These references describe the application of the technique to biologically important molecules. In both of these references the spectrometer is constructed so that the desorption-inducing .sup.252 Cf fission fragment must penetrate the supporting metal foil and the entire thickness of the sample. Heretofore all fission fragment induced desorption TOFMS instruments have had this limitation.